1. Field of the Invention
This invention relates generally to endpoint detection in a chemical mechanical polishing process, and more particularly to endpoint detection using optical interference of a broad reflectance spectrum.
2. Description of the Related Art
In the fabrication of semiconductor devices, typically, the integrated circuit devices are in the form of multi-level structures. At the substrate level, transistor devices having diffusion regions are formed. In subsequent levels, interconnect metallization lines are patterned and electrically connected to the transistor devices to define the desired functional device. As is well known, patterned conductive layers are insulated from other conductive layers by dielectric materials, such as silicon dioxide. As more metallization levels and associated dielectric layers are formed, the need to planarize the dielectric material increases. Without planarization, fabrication of additional metallization layers becomes substantially more difficult due to the higher variations in the surface topography. In other applications, metallization line patterns are formed in the dielectric material, and then metal chemical mechanical polishing (CMP) operations are performed to remove excess metallization.
In the prior art, CMP systems typically implement belt, orbital, or brush stations in which belts, pads, or brushes are used to scrub, buff, and polish one or both sides of a wafer. Slurry is used to facilitate and enhance the CMP operation. Slurry is most usually introduced onto a moving preparation surface, e.g., belt, pad, brush, and the like, and distributed over the preparation surface as well as the surface of the semiconductor wafer being buffed, polished, or otherwise prepared by the CMP process. The distribution is generally accomplished by a combination of the movement of the preparation surface, the movement of the semiconductor wafer and the friction created between the semiconductor wafer and the preparation surface.
FIG. 1A shows a cross sectional view of a dielectric layer 102 undergoing a fabrication process that is common in constructing damascene and dual damascene interconnect metallization lines. The dielectric layer 102 has a diffusion barrier layer 104 deposited over the etch-patterned surface of the dielectric layer 102. The diffusion barrier layer, as is well known, is typically titanium nitride (TiN), tantalum (Ta), tantalum nitride (TaN) or a combination of tantalum nitride (TaN) and tantalum (Ta). Once the diffusion barrier layer 104 has been deposited to the desired thickness, a copper layer 106 is formed over the diffusion barrier layer in a way that fills the etched features in the dielectric layer 102. Some excessive diffusion barrier and metallization material is also inevitably deposited over the field areas. In order to remove these overburden materials and to define the desired interconnect metallization lines and associated vias (not shown), a chemical mechanical planarization (CMP) operation is performed.
As mentioned above, the CMP operation is designed to remove the top metallization material from over the dielectric layer 102. For instance, as shown in FIG. 1B, the overburden portion of the copper layer 106 and the diffusion barrier layer 104 have been removed. As is common in CMP operations, the CMP operation must continue until all of the overburden metallization and diffusion barrier material 104 is removed from over the dielectric layer 102. However, in order to ensure that all the diffusion barrier layer 104 is removed from over the dielectric layer 102, there needs to be a way of monitoring the process state and the state of the wafer surface during its CMP processing. This is commonly referred to as endpoint detection. Endpoint detection for copper is performed because copper cannot be successfully polished using a timed method. A timed polish does not work with copper because the removal rate from a CMP process is not stable enough for a timed polish of a copper layer. The removal rate for copper from a CMP process varies greatly. Hence, monitoring is needed to determine when the endpoint has been reached. In multi-step CMP operations there is a need to ascertain multiple endpoints: (1) to ensure that Cu is removed from over the diffusion barrier layer; (2) to ensure that the diffusion barrier layer is removed from over the dielectric layer. Thus, endpoint detection techniques are used to ensure that all of the desired overburden material is removed.
Many approaches have been proposed for the endpoint detection in CMP of metal. The prior art methods generally can be classified as direct and indirect detection of the physical state of polish. Direct methods use an explicit external signal source or chemical agent to probe the wafer state during the polish. The indirect methods on the other hand monitor the signal internally generated within the tool due to physical or chemical changes that occur naturally during the polishing process.
Indirect endpoint detection methods include monitoring: the temperature of the polishing pad/wafer surface, vibration of polishing tool, frictional forces between the pad and the polishing head, electrochemical potential of the slurry, and acoustic emission. Temperature methods exploit the exothermic process reaction as the polishing slurry reacts selectively with the metal film being polished. U.S. Pat. No. 5,643,050 is an example of this approach. U.S. Pat. No. 5,643,050 and U.S. Pat. No. 5,308,438 disclose friction-based methods in which motor current changes are monitored as different metal layers are polished.
Another endpoint detection method disclosed in European application EP 0 739 687 A2 demodulates the acoustic emission resulting from the grinding process to yield information on the polishing process. Acoustic emission monitoring is generally used to detect the metal endpoint. The method monitors the grinding action that takes place during polishing. A microphone is positioned at a predetermined distance from the wafer to sense acoustical waves generated when the depth of material removal reaches a certain determinable distance from the interface to thereby generate output detection signals. All these methods provide a global measure of the polish state and have a strong dependence on process parameter settings and the selection of consumables. However, none of the methods except for the friction sensing have achieved some commercial success in the industry.
Direct endpoint detection methods monitor the wafer surface using acoustic wave velocity, optical reflectance and interference, impedance/conductance, electrochemical potential change due to the introduction of specific chemical agents. U.S. Pat. No. 5,399,234 and U.S. Pat. No. 5,271,274 disclose methods of endpoint detection for metal using acoustic waves. These patents describe an approach to monitor the acoustic wave velocity propagated through the wafer/slurry to detect the metal endpoint. When there is a transition from one metal layer into another, the acoustic wave velocity changes and this has been used for the detection of endpoint. Further, U.S. Pat. No. 6,186,865 discloses a method of endpoint detection using a sensor to monitor fluid pressure from a fluid bearing located under the polishing pad. The sensor is used to detect a change in the fluid pressure during polishing, which corresponds to a change in the shear force when polishing transitions from one material layer to the next. Unfortunately, this method is not robust to process changes. Further, the endpoint detected is global, and thus the method cannot detect a local endpoint at a specific point on the wafer surface. Moreover, the method of the U.S. Pat. No. 6,186,865 patent is restricted to a linear polisher, which requires an air bearing.
There have been many proposals to detect the endpoint using the optical reflectance from the wafer surface. They can be grouped into two categories: monitoring the reflected optical signal at a single wavelength using a laser source or using a broad band light source covering the full visible range of the electromagnetic spectrum. U.S. Pat. No. 5,433,651 discloses an endpoint detection method using a single wavelength in which an optical signal from a laser source is impinged on the wafer surface and the reflected signal is monitored for endpoint detection. The change in the reflectivity as the polish transfers from one metal to another is used to detect the transition.
Broad band methods rely on using information in multiple wavelengths of the electromagnetic spectrum. U.S. Pat. No. 6,106,662 discloses using a spectrometer to acquire an intensity spectrum of reflected light in the visible range of the optical spectrum. Two bands of wavelengths are selected in the spectra that provide good sensitivity to reflectivity change as polish transfers from one metal to another. A detection signal is then defined by computing the ratio of the average intensity in the two bands selected. Significant shifts in the detection signal indicate the transition from one metal to another.
A common problem with current endpoint detection techniques is that some degree of over-etching is required to ensure that all of the conductive material (e.g., metallization material or diffusion barrier layer 104) is removed from over the dielectric layer 102 to prevent inadvertent electrical interconnection between metallization lines. A side effect of improper endpoint detection or over-polishing is that dishing 108 occurs over the metallization layer that is desired to remain within the dielectric layer 102. The dishing effect essentially removes more metallization material than desired and leaves a dish-like feature over the metallization lines. Dishing is known to impact the performance of the interconnect metallization lines in a negative way, and too much dishing can cause a desired integrated circuit to fail for its intended purpose.
In view of the foregoing, there is a need for endpoint detection systems and methods that improve accuracy in endpoint detection. In addition, the systems and methods should be able to accurately determine film and layer thickness.
Broadly speaking, the present invention fills these needs by providing an optical endpoint detection system that utilizes optical interference to determine when an endpoint has been reached, such as when metal layer has reached a thin metal zone during a CMP process. In one embodiment, a method for detecting an endpoint during a chemical mechanical polishing process is disclosed. A portion of the surface of a wafer is illuminated with broad band light source. Then, reflected spectrum data corresponding to a plurality of spectrums of light reflected from the illuminated portion of the wafer surface is received. An endpoint is then determined based on optical interference occurring in the reflected spectrum data. The optical interference is a result of phase differences in light reflected from different layers of the wafer, and occurs when the top metal layer is reduced to the thin metal zone. Optical interference is indicated by the oscillations in the reflected spectrum data, and the endpoint occurs when the oscillations appear in the reflected spectrum. To detect the occurrences of the oscillations, Fourier Transform is applied on the reflected spectrum, and the peak magnitudes of the Fourier Transform within a specified thickness window are summed. The endpoint occurs when the sum of peak magnitudes exceeds a predetermined threshold.
In another embodiment, an endpoint detection apparatus is disclosed that detects an endpoint during a chemical mechanical polishing process. The endpoint detection apparatus includes a broad band light source for illuminating a portion of a surface of a wafer, and an optical detector for receiving reflected spectrum data corresponding to a plurality of spectrums of light reflected from the illuminated portion of the wafer surface. The endpoint detection apparatus further includes logic that determines an endpoint based on optical interference occurring in the reflected spectrum data. As, discussed above the optical interference is a result of phase differences in light reflected from different layers of the wafer, and occurs when the top metal layer is reduced to the thin metal zone.
A system for detecting an endpoint during a chemical mechanical polishing process is disclosed in a further embodiment of the present invention. The system includes a polishing pad having a pad slot, and a platen having a platen slot. The platen slot is designed to align with the pad slot during particular points of the chemical mechanical polishing process. The system also includes a broad band light source for illuminating a portion of a surface of a wafer through the platen slot and the pad slot. An optical detector receives reflected spectrum data corresponding to a plurality of spectrums of light reflected from the illuminated portion of the surface of the wafer. Further, logic is provided that determines an endpoint based on optical interference occurring in the reflected spectrum data.